Dehydroisomerisation catalyst and its use in the preparation of isobutene from N-butane

ABSTRACT

A catalyst is described for the preparation of isobutene by dehydroisomerization of n-butane consisting of a solid, granular support of porous gamma-alumina on the surface of which catalytic quantities of platinum, silica and preferably also one or more promotors are deposited. In a preferred form of embodiment this catalyst is used with a second catalyst which may be formed of Boralite B or a solid granular support of gamma-alumina on the surface of which catalytic quantities of silica are deposited. The present invention relates also to the process of dehydroisomerization of n-butane to isobutene in which the catalyst is used and the relative operating conditions.

This is a divisional of application Ser. No. 07/737,067, filed Jul. 29,1991 now U.S. Pat. No. 5,275,995.

DESCRIPTION

The present invention relates to a catalyst for the preparation ofisobutene by dehydroisomerisation of n-butane and the process that usesthe said catalyst.

Isobutene is a valuable intermediate used in various chemical reactions,for example as a monomer in polymerization and copolymerizationreactions, as an alkylation agent in the production of methyl tert-butylether ( reaction with methanol) and in the production of isoprene(reaction with formaldehyde).

Isobutene is usually obtained as a by-product in refining processes suchas thermal or catalytic cracking, or by means of a two-stage process inthe first of which n-butane is isomerised catalytically to isobutane andthe latter is dehydrogenated catalytically to isobutene.

Catalytic processes have recently been described which in a singlereaction stage enable the conversion of n-alkanes into iso-olefines. Inparticular, in EP 42.252, isobutene is obtained by thedehydroisomerisation of n-butane in the presence of a catalyst formed byan element of Group IIIa of the Periodic Table, or the relative salt,and from a low-acidity support. In U.S. Pat. No. 4.433.190 thedehydroisomerisation reaction of an n-alkane into the correspondingiso-olefine is performed on a catalyst obtained by impregnating a porouscrystalline boro-silicate called AMS-1B with a noble metal. Thesedehydroisomerisation processes fail to produce totally satisfactoryresults, particularly from the point of view of the limited yield of thedesired product. There are also known state-of-the-art dehydrogenationcatalysts comprising a support on which platinum or another noble metalhas been deposited. These catalysts may contain promoters usually chosenamong tin, indium,thallium or one of the alkaline-earth metals. Theselatter catalysts are generally used in dehydrogenation processes oflong-chain paraffins, as described in U.S. Pat. No. 4,486,547 and inItalian Application for Patent IT 23.149/87, filed on Dec. 22, 1987.

In BE 877205 a porous crystalline boro-silicate named Boralite B isdescribed capable also of isomerising n-paraffin.

In U.S. Pat. No. 4,038,337 a process for the isomerisation of alkenes toiso-alkenes is described which uses a catalyst of alumina containingsilica on its surface.

It is also known that n-butane can be dehydroisomerised to isobutene ina single stage using two-component catalytic systems, in which the firstcomponent is alumina onto which catalytic quantities of platinum andpossibly one or more promotors are absorbed and the second component maybe alumina on whose surface catalytic quantities of silica are depositedor may be Boralite B.

A new catalyst has now been found, comprising alumina which on itssurface contains platinum, silica and preferably one or more promotors,which in suitable reaction conditions enables dehydroisomerisation ofn-butane to isobutene in a single stage giving high yields of usefulproduct.

Thus the prime object of the present invention is a catalyst (a) formedof a solid support of porous gamma-alumina on the surface of which aredeposited catalytic quantities of platinum, silica and preferably, aspromotors, tin and/or indium.

In a preferred form of embodiment this catalyst is used with a secondcatalyst (b) which may be formed of Boralite B or a granular solidsupport of porous gamma-alumina on the surface of which are depositedcatalytic quantities of silica.

The support required for catalyst (a) is alumina in the gammacrystallograhic form, which has a surface area of 100 to 400 m² /g and atotal volume of pores of 0.5 to 1.2 ml/g, in the form of granules,extrusions or pellets with a useful size for use in a fixed catalyticbed and generally varying from 0. 4 to 5 mm.

Catalyst (a), required for the aims of the present invention, is formedof a gamma with the above-described characteristics, on which aredeposited platinum in a quantity of 0.1 to 1% by weight and silica in aquantity of 0.5 to 5% by weight, preferably 1-2.5%.

Catalyst (a) preferably contains, as promotors, also tin in a quantityof 0.1 to 1% by weight and/or indium in a quantity of 0.05 to 1%. Inthis case the following weight ratios are suitably maintained:platinum/indium from 0.3:1 to 1.5:1 and platinum/tin from 0.5:1 to 2:1.All of the above percentages refer to the total weight of the catalyst.Catalyst (a) is prepared by means of a two-stage process consisting in:

1) thermally decomposing an alkyl ortho-silicate in the presence ofgamma-alumina and treating the resulting product at high temperature,first in an inert atmosphere and then in an oxidizing atmosphere;

2) impregnating the silicified alumina obtained in the previous stepwith an acid aqueous solution, especially acidified with nitric acid, ofa compound of platinum, and possibly tin and/or indium, then drying andcalcining.

As regards the first step, relating to the preparation of silicifiedalumina, described and claimed in U.S. Pat. No. 4,013,590, suitablequantities of gamma-alumina and alkyl ortho-silicate, preferably ethylortho-silicate, are placed in contact with each other at an operatingtemperature of 100 to 400° C., at a pressure of 10 to 30 kg/cm² and fora contact time of approx. 1 to 20 hours. This treatment is conducted inthe absence of oxygen and humidity, presurizing with an inert gas,preferably nitrogen, at a temperature of 100 to 400° C., for a timevarying of from 1 to 4 h, and then in an air flow at a temperature of400° to 600° C. for a time of from 2 to 8 hours. As regards the secondstep in particular, hydrosoluble and high temperatures decomposableplatinum, and possibly tin and indium, compounds, such as chloroplatinicacid, stannic chloride, and indium nitrate, are used for impregnation.

Drying of the impregnated support is best performed at temperatures inthe order of 100°-130° C. in an air flow and calcination is best carriedout at 400°-600° C., in an air flow for a time in the order of 2-8hours. The catalyst thus obtained is subjected to a reduction treatmentbefore use in the dehydroisomerisation reaction. This treatment, whichcan be carried out in the dehydroisomerisation reactor, is usuallyperformed in a flow of hydrogen, at high temperatures (approx. 450°-650°C.) for a time in the order of 1-5 hours.

This catalyst is used in the dehydroisomerisation of n-butane toisobutene preferably with a second catalyst (b) formed of Boralite B ora granular solid support of porous gamma-alumina on the surface of whichare deposited catalytic quantities of silica.

When catalyst (b) is silicified gamma-alumina, it is prepared byfollowing the same procedure previously described in step 1) relating tothe preparation of catalyst (a) . Catalyst (b) thus obtained issubjected to a reduction treatment before its use in thedehydroisomerisation reaction. In the most preferred form, catalysts (a)and (b) are simultaneously subjected to such reduction treatment.Catalyst (b) may also be Boralite B. Boralite B is a zeolite describedin BE 877205. In the anhydrous and calcined form it has the followingmolar composition with the components expressed as oxides:

    C.sub.2/n 0.B.sub.2 O.sub.3.(5-50)SiO.sub.2

where

C=metallic cation of valence n, H⁺, NH₄ ⁺, or a mixture of these.

Boralite B is prepared as described in BE 877205, by reaction inhydrothermal conditions with a derivative of silicon, a derivative ofboron, an hydroxide of an alkaline metal and a salt of tetra-ethylammonium. More particularly, a derivative of silicon chosen for examplefrom colloidal silica, silica gel or sodium silicate, a derivative ofboron chosen for example from boric acid, alkaline borates or tri-alkylborates, an hydroxide of alkaline metal and a salt of tetra-ethylammonium, preferably tetra-ethyl ammonium hydroxide, are heated in anautoclave at an autogenous pressure and at a temperature of between 90°C. and 160° C., until complete crystallisation. The crystalline productthus obtained is then filtered, dried and calcined.

In particular, crystallisation preferably occurs in the presence ofgrains of Boralite B, in a quantity of between 1 and 60% by weight,which reduce the time required for synthesis.

These grains are obtained by placing a derivative of silicon, aderivative of boron, an hydroxide of an alkaline metal and a salt oftetra-alkyl ammonium to react in an autoclave, in the same ratiosdescribed in BE 877205, under hydrothermal conditions, for a period ofat least one day. Boralite B is used in the present invention in theform of granules, extrusions or pellets of a suitable size for use

in a fixed catalytic bed, generally variable between 0.4 and 5 mm.

A second object of the present invention is the process for preparingisobutene by dehydroisomerisation of n-butane making use of theabove-mentioned catalyst (a), where such catalyst is used as it is orpreferably with catalyst (b) described above.

The process according to the invention consists in supplying a gaseousmixture of n-butane and hydrogen, possibly diluted with an inert gas, toa fixed catalytic bed comprising catalyst (a), as it is or preferablywith catalyst (b).

In the gaseous supply flow a molar ratio between the hydrogen andn-butane of 1:1 to 5:1 should be maintained and preferably of 1:1 to3:1. If the gaseous supply flow is diluted, for example with nitrogen,the molar ratios between hydrogen and n-butane become comprised withinthe range of from 1:1 to 5:1 and that between nitrogen and n-butanewithin the range of from 1:1 to 5:1 and preferably of from 1:1 to 3:1.

The dehydroisomerisation reaction is conducted within a temperaturerange of 450° to 600° C., at a pressure of 200 mm Hg to 5 kg/cm² and atan hourly spatial speed of 0.5 to 5 hours⁻¹ (n-butane weight/catalystweight.hour).

Preferably the temperatures vary from 500° to 580° C., the pressuresvary from 400 mm Hg to 2 kg/cm² and the spatial speed is between 2 and 4hours⁻¹.

When catalyst (a) is used with catalyst (b) the said catalysts (a) and(b) are homogeneously distributed onto the catalytic bed, or arearranged in the form of two contiguous layers.

In this second case the layer of catalyst (a) will be arranged in thedehydroisomerisation reactor so as to come into contact firstly with thegaseous supply flow. The catalytic bed will also contain catalysts (a)and (b) in weight ratios between them of 20:80 to 80:20, preferably inthe order of 70:30.

By operating in accordance with the process covered by the presentinvention high conversions of the n-butane supplied

are obtained, with high yields and selectivity of the useful reactionproduct.

The experimental examples that follow are given as further illustrationof the invention.

EXAMPLE 1

Preparing Catalyst (a)

The preparation of silicified alumina in this and the following examplesis achieved in accordance with U.S. Pat. No. 4,013,590. For thepreparation of catalyst (a) a commercial gamma-alumina is used, having asurface area of 196 m² /g and a total volume of pores of 0.75 ml/g, inthe form of granules of a size of 0.5-0.8 mm. 20 g of this gamma-aluminaare placed in an autoclave together with 1.5 g of ethyl ortho-silicate.This is left to rest for 2 hours, then the autoclave is emptied toremove any excess ethyl ortho-silicate which has not reacted, washedwith nitrogen to exclude the presence of oxygen and lastly brought to apressure of 5 kg/cm² using nitrogen. The autoclave is heated to 200° C.and kept at that temperature for 4 hours. At the end of that period,after cooling, the pressure is discharged and the solid recovered thensubjected to further heat treatment of 2 hours at 200° C. in nitrogenand to calcination in air at 500° C. for 4 hours. Lastly after cooling,the solid consisting of gamma-alumina containing a later of silica onits surface, is recovered in a quantity of 1.5% by weight. To 20 g ofthis solid 1.9 g of chloroplatinic acid is slowly added under agitation(to 4.21% by weight of platinum). After 12 hours of contact at ambienttemperature the mass is heated to 120° C., in an air flow, for 1 hour,until the sample is dried. The dried solid thus obtained is calcined ina muffle at 500° C. for 4 hours in an air flow. Finally the autoclave iscooled and the catalyst (a) which is recovered, contains 0.4% by weightof platinum.

EXAMPLE 2

Preparing Catalyst (a) with Promotors

A commercial gamma-alumina is used having a surface area of 196 m² /gand a total volume of pores of 0.75 ml/g, in the form of granules of asize of 0.5-0.8 mm. 20g of this gamma-alumina are placed in an autoclavetogether with 1.5 g of ethyl ortho-silicate. This is left to rest for 2hours, then the autoclave is emptied to remove any excess ethylortho-silicate which has not reacted, washed with nitrogen to excludethe presence of oxygen and lastly brought to a pressure of 5 kg/cm²using nitrogen. The autoclave is heated to 200° C. and kept at thattemperature for 4 hours. At the end of that period after cooling, thepressure is discharged and the solid recovered and then subjected tofurther heat treatment of 2 hours at 200° C. in nitrogen and tocalcination in air at 500° C. for 4 hours. Lastly, after cooling, thesolid consisting of gamma-alumina containing a layer of silica on itssurface, is recovered in a quantity of 1.5% by weight.

To 20 g of this gamma-alumina 3 ml of an aqueous solution obtained from0.25 g of nitrate of indium pentahydrate, 0.2 g of stannic chloride,0.47 g of chloroplatinic acid (to 16% by weight of platinum) and 1.3 gof nitric acid at 65% is slowly added under agitation. After one hour ofcontact at ambient temperature (about 25° C.), under continuousagitation, the mass is heated to about 120° C., in an air flow, for 1hour, until the excess aqueous solvent is virtually completelyevaporated. The dried solid thus obtained is calcined in a muffle at500° C. for 4 hours in an air flow. Finally the autoclave is cooled andthe catalyst (a) which is recovered, contains 0.37% by weight ofplatinum, 0.50% by weight of tin and 0.36% by weight of indium.

EXAMPLE 3

For the preparation of catalyst (b) a commercial gamma-alumina is usedhaving a surface area of 196 m² /g and a total volume of pores of 0. 75ml/g, in the form of granules of a size of 0.5-0.8mm. 20 g of thisgamma-alumina are placed in an autoclave together with 1.5 g of ethylortho-silicate. This is left to rest for 2 hours, then the autoclave isemptied to remove any excess ethyl ortho-silicate which has not reacted,washed with nitrogen to exclude the presence of oxygen and lastlybrought to a pressure of 5 kg/cm² using nitrogen. The autoclave isheated to 200° C. and kept at that temperature for 4 hours. At the endof that period after cooling, the pressure is discharged and the solidrecovered and subjected to further heat treatment of 2 hours at 200° C.in nitrogen and to calcination in air at 500° C. for 4 hours. Lastly,after cooling, the solid consisting of gamma-alumina containing a layerof silica on its surface, is recovered in a quantity of 1.5% .by weight.

EXAMPLE 4

Preparation of Boralite B

In 28.12 g of an aqueous solution of tetra-ethyl ammonium hydroxide at40% by weight, 3.0 g of NaOH and 6.4 g of boric acid are dissolved. Aclear solution is obtained which is diluted with 30g of distilled waterand added to 51g of Ludox AS silica at 30% by weight of silica.

The suspension thus obtained, having a pH of 12.2, is left at ambienttemperature under agitation for 4 hours and then placed to crystallisein an autoclave, in static conditions, at autogenous pressure, at 150°C., for 5 days.

The autoclave is then cooled and the milky suspension of grains ofBoralite B is recovered. This suspension is added in quantities equal to15% by weight to a mixture having the following composition, after thelatter has been kept in agitation at ambient temperature for about 4hours:

112.5 g of TEA-OH at 40% in water

12.0 g of NaOH

25.5 g of H₃ BO₃

120.0 g Of distilled water

204 g of Ludox AS silica at 30% by weight.

This mixture added to the suspension of grains is placed to crystallisein a steel autoclave under static conditions at an autogenous pressure,at a temperature of 150° C., for 3 days.

The autoclave is cooled, the Boralite B is recovered by filtration, itis washed with distilled water, dried at 120° C., calcined for 5 hoursat 550° C. and then exchanged into acid form according the knownstate-of-the-art methods. The Boralite B thus obtained, having crystalsof a size of around 1 μm, is made into pellets with a granular size of0.4 to 0.8 mm.

EXAMPLE 5

0.57 g of catalyst (a) prepared as described in Example 1 are placed ina quartz reactor with an internal diameter of 10 mm. The catalyst issubjected to in situ reduction, by supplying a flow of hydrogen for 2hours, at a temperature of 550° C.

After this treatment the dehydroisomerisation test is performed bysupplying the reactor with a gaseous mixture containing n-butane,hydrogen and nitrogen, with a hydrogen/n-butane molar ratio of 1:1 and anitrogen/n-butane molar ratio of 2:1. In addition the test is conductedat a temperature of 553° C. at atmospheric pressure and with an hourlyspatial speed of 2 hours⁻¹ (n-butane weight/catalyst weight.hour).

The results of the test are given in Table 1.

EXAMPLE 6

0.34 g of catalyst (a) prepared as described in Example 2 are placed ina quartz reactor with an internal diameter of 10 mm. The catalyst issubjected to in situ reduction, by supplying a flow of hydrogen for 2hours, at a temperature of 550° C. After this treatment thedehydroisomerisation test is performed by supplying the reactor with agaseous mixture containing n-butane, hydrogen and nitrogen, with ahydrogen/n-butane molar ratio of 1:1 and a nitrogen/n-butane molar ratioof 2:1. In addition the test is conducted at a temperature of 563° C. atatmospheric pressure and with an hourly spatial speed of 2 hours(n-butane weight/catalyst weight.hour).

The results of the test are given in Table 1.

EXAMPLE 7

0.32 g of catalyst prepared as described in Example 2 are placed in aquartz reactor with an internal diameter of 10 mm. The catalyst issubjected to in situ reduction, by supplying a flow of hydrogen for 2hours, at a temperature of 550° C.

After this treatment the dehydroisomerisation test is performed bysupplying the reactor with a gaseous mixture containing n-butane,hydrogen and nitrogen, with a hydrogen/n-butane molar ratio of 1:1 and anitrogen/n-butane molar ratio of 2:1. In addition the test is conductedat a temperature of 560° C. at atmospheric pressure and with an hourlyspatial speed of 4 hours⁻¹ (n-butane weight/catalyst weight.hour).

The results of the test are given in Table 1.

EXAMPLE 8

0.33 g of catalyst (a) prepared as described in Example 2 and 0.31 g ofcatalyst (b) prepared as described in Example 3, are placed separatelyin a quartz reactor with an internal diameter of 10mm.

The catalysts are subjected to in situ reduction, by supplying a flow ofhydrogen for 2 hours, at a temperature of 550° C.

After this treatment the dehydroisomerisation test is performed bysupplying the reactor with a gaseous mixture containing n-butane,hydrogen and nitrogen, with a hydrogen/n-butane molar ratio of 1:1 and anitrogen/n-butane molar ratio of 2:1. In addition the test is conductedat a temperature of 563° C. at atmospheric pressure and with an hourlyspatial speed, calculated for catalyst (a) only, of 2 hours⁻¹ (n-butaneweight/catalyst weight.hour).

The results of the test are given in Table 1.

EXAMPLE 9

0.50 g of catalyst (a) prepared as described in Example 2 and 0.25 g ofcatalyst (b) prepared as described in Example 4, are mixed untilhomogeneity and then placed in the form of a fixed bed in a quartzreactor with an internal diameter of 10 mm.

The catalysts are subjected to in situ reduction, by supplying a currentof hydrogen for 2 hours, at a temperature of 550° C.

After this treatment the dehydroisomerisation test is performed bysupplying the reactor with a gaseous mixture containing n-butane,hydrogen and nitrogen, with a hydrogen/n-butane molar ratio of 1:1 and anitrogen/n-butane molar ratio of 2:1. In addition the test is conductedat a temperature of 564° C. at atmospheric pressure and with an hourlyspatial speed, calculated for catalyst (a) only, of 2 hours⁻¹ (n-butaneweight/catalyst weight-hour).

The results of the test are given in Table 1.

EXAMPLE 10

0.86 g of catalyst (a) prepared as described in Example 2 and

0.33 g of catalyst (b) prepared as described in Example 4, are placedseparately in a quartz reactor with an internal diameter of 10 mm andsubjected to preliminary reduction in a flow of hydrogen at atemperature of 550° C., for 2 hours. After reduction thedehydroisomerisation test is performed by supplying the reactor with agaseous mixture containing hydrogen, n-butane, and nitrogen, with ahydrogen/n-butane molar ratio of 1:1 and a nitrogen/n-butane molar ratioof 2:1. In addition the test is conducted at a temperature of 552° C. atatmospheric pressure and with an hourly spatial speed, assessed oncatalyst (a), of 2 hours⁻¹ (n-butane weight/catalyst weight.hour).

The results of the test are given in Table 1.

EXAMPLE 11

0.86 g of catalyst (a) prepared as described in Example 2 and 0.33 g ofcatalyst (b) prepared as described in Example 4, are placed separatelyin a quartz reactor with an internal diameter of 10 mm and subjected topreliminary reduction in a flow of hydrogen at a temperature of 550° C.,for 2 hours. After reduction the dehydroisomerisation test is performedby supplying the reactor with a gaseous mixture containing hydrogen,n-butane, and nitrogen, with a hydrogen/n-butane molar ratio of 1:1 anda nitrogen/n-butane molar ratio of 2:1. In addition the test isconducted at a temperature of 551° C. at atmospheric pressure and withan hourly spatial speed, assessed on catalyst (a), of 4 hours⁻¹(n-butane weight/catalyst weight.hour).

The results of the test are given in Table 1.

Example 12 (Comparison)

A crystalline borosilicate, called AMS-1B, is prepared as described inExample 1 of U.S. Pat. No. 4,269,813. More particularly, 10.5 g of boricacid and 67.2 g of sodium hydroxide are dissolved in 2,653 g of waterunder continuous agitation. To the solution thus obtained 394.8 g oftetra-n-propylammonium bromide and, after completely dissolving, 400 gof Ludox commercial silica are added. The solution is placed in anautoclave and left to crystallise at 165° C. for 7 days. The crystallinesolid thus obtained is dried and calcined at 550° C. for 5 hours,exchanged into acid form and again dried and calcined. A sample of 4 gof this solid, in granules with a size of 0.5 to 0.8 mm, is impregnatedwith a solution of chloroplatinic acid at 16% by weight, so as to obtaina metallic platinum content of 0.55% by weight. Lastly, the solid isdried and calcined for 12 hours at 350° C. to obtain the catalyst. 0.34g of the catalyst prepared as described above are placed in the quartzmicroreactor. Reduction is performed in a flow of hydrogen, at 525° C.for 2 hours. After this treatment the dehydroisomerisation test isperformed. The supply to the reactor comprises n-butane, hydrogen andnitrogen, with a hydrogen/n-butane molar ratio of 1:1 and anitrogen/n-butane molar ratio of 2:1. The reaction is performed at atemperature of 542° C., at atmospheric pressure and with an hourlyspatial speed of 8 hours⁻¹. The results of the test are shown in Table1.

Example 13 (Comparison)

A sample of AMS-1B crystalline borosilicate prepared as described inExample 7, is mixed with 16 g of gamma-alumina. The mixture isgranulated until the granular size is 0.5-0.8 mm and the granules areimpregnated with a chloroplatinic acid solution at 4.1% by weight,according to Example 9 of U.S. Pat. No. 4,433,190. The impregnated solidis dried and calcined at 350° C. for 12 hours. 0.3 g of the catalystthus obtained are placed in a quartz reactor. After the reductiontreatment, performed in a flow of hydrogen at a temperature of 525° C.for two hours, the dehydroisomerisation test is performed. The supplycomprises n-butane, hydrogen and nitrogen, with a hydrogen/n-butanemolar ratio of 1:1 and a nitrogen/n-butane molar ratio of 2:1. Thereaction is performed at a temperature of 542° C., at atmosphericpressure and with an hourly spatial speed of 8 hours⁻¹.

The results of the test are given in Table 1.

In this Table, conversion means the percentage by weight of n-butaneconverted as compared to that supplied. Furthermore, the selectivity andyield relate to the converted reagent and supplied reagent respectively.Lastly, C1-C3 and C5⁺ indicate the paraffinic and olefinic productsrespectively containing from 1 to 3 carbon atoms and those with 5 ormore carbon atoms.

                                      TABLE 1                                     __________________________________________________________________________              Example No.                                                                   5   6  7   8  9  10 11 12  13                                       __________________________________________________________________________    Conversion (%)                                                                          45.1                                                                              46.3                                                                             56.3                                                                              56.8                                                                             57.8                                                                             58.4                                                                             58.5                                                                             8.1 33.7                                     Selectivity (%) into:                                                         i-butene  20.0                                                                              21.1                                                                             20.6                                                                              23.3                                                                             22.1                                                                             25.0                                                                             24.5                                                                             29.6                                                                              11.8                                     n-butenes 49.3                                                                              56.8                                                                             57.4                                                                              41.2                                                                             45.7                                                                             39.9                                                                             44.4                                                                             56.1                                                                              73.6                                     Total C4s 73.4                                                                              81.2                                                                             86.7                                                                              71.9                                                                             76.9                                                                             76.1                                                                             82.4                                                                             85.7                                                                              85.4                                     Yield (%) in:                                                                 C1-C3     7.8 3.6                                                                              3.1 8.2                                                                              5.3                                                                              6.8                                                                              5.3                                                                              0.8 3.7                                      C5.sup.+  Traces                                                                            1.2                                                                              Traces                                                                            1.6                                                                              1.8                                                                              2.8                                                                              1.9                                                                              0.3 1.2                                      i-butene  9.0 9.8                                                                              11.6                                                                              13.2                                                                             12.8                                                                             14.6                                                                             14.3                                                                             2.4 4.0                                      aromatics 2.7 1.9                                                                              1.7 5.5                                                                              4.7                                                                              4.4                                                                              2.9                                                                              traces                                                                            0.7                                      __________________________________________________________________________

We claim:
 1. A process for the preparation of isobutene bydehydroisomerisation of n-butane, comprising contacting, underdehydroisomerization conditions, a gaseous flow of n-butane andhydrogen, at 450°-600° C., with a fixed catalytic bed formed of a solidgranular support of porous gamma-alumina on the surface of which aredeposited, as the catalyst, 0.1 to 1% by weight of platinum and 0.5 to5% by weight of silica.
 2. A process for the preparation of isobutane bydehydroisomerization of n-butane, comprising contacting, underdehydroisomerization conditions, a gaseous flow of n-butane andhydrogen, at 450°-650° C. with a catalytic system comprising a firstcatalyst which is formed of a solid granular support of porousgamma-alumina on the surface of which are deposited, as the catalyst,0.1 to 1% by weight of platinum and 0.5 to 5% by weight of silica and asecond catalyst formed of a porous gamma-alumina on the surface of whichis deposited a catalytic quantity of silica.
 3. A process for thepreparation of isobutene by dehydroisomerization of n-butane, comprisingcontacting, under dehydroisomerization conditions, a gaseous flow ofn-butane and hydrogen at 450°-600° C. with a catalytic system comprisinga first catalyst which is formed of a solid granular support of porousgamma-alumina on the surface of which are deposited, as the catalyst,0.1 to 1% by weight of platinum and 0.5 to 5% by weight of silica and asecond catalyst formed of Boralite B having in its anhydrous or calcinedform the following molar composition with the components expressed asoxides:

    C.sub.2/n 0B.sub.2 O.sub.3 (5-50)SiO.sub.2

where C is selected from the group consisting of a metallic cation ofvalence n, H⁺, NH₄ ⁺, and mixtures thereof.
 4. The process of claim 2,wherein the weight ratio between the first and second catalysts variesfrom 20:80 to 80:20.
 5. The process of claim 4, wherein the weight ratiois 70:30.
 6. The process of claim 2, wherein the catalysts arehomogeneously distributed in the catalytic bed or are arranged in theform of two contiguous layers, with the layer of the first catalystbeing contacted first by the gaseous flow of n-butane and hydrogen. 7.The process of claim 3, wherein the weight ratio between the first andsecond catalysts varies from 20:80 to 80:20 and, wherein the catalystsare homogeneously distributed in the catalytic bed or are arranged inthe form of two contiguous layers, with the layer of the first catalystbeing contacted first by the gaseous flow of n-butane and hydrogen. 8.The process of claim 1, wherein in the gaseous flow a molar ratiobetween hydrogen and n-butane is maintained between 1:1 and 5:1.
 9. Theprocess of claim 2, wherein in the gaseous flow, a molar ratio betweenhydrogen and n-butane is maintained between 1:1 and 5:1.
 10. The processof claim 3, wherein in the gaseous flow, a molar ratio between hydrogenand n-butane is maintained between 1:1 and 5:1.
 11. The process of claim1, wherein in the gaseous flow, a molar ratio between hydrogen andn-butane is maintained between 1:1 and 3:1.
 12. The process of claim 2,wherein in the gaseous flow, a molar ratio between hydrogen and n-butaneis maintained between 1:1 and 3:1.
 13. The process of claim 1, whereinthe gaseous flow of n-butane and hydrogen is diluted with nitrogenbefore being placed in contact with the fixed catalytic bed.
 14. Theprocess of claim 2, wherein the gaseous flow, a molar ratio betweenhydrogen and n-butane is maintained between 1:1 and 3:1 and, wherein thegaseous flow of n-butane and hydrogen is diluted with nitrogen beforebeing placed in contact with the fixed catalytic system.
 15. The processof claim 3, wherein the gaseous flow of n-butane and hydrogen is dilutedwith nitrogen before being placed in contact with the fixed catalyticsystem.
 16. The process of claim 1, wherein the dehydroisomerizationreaction is conducted at 450° to 600° C., at a pressure of 200 mm. Hg to5 kg/cm² and with an hourly spatial speed of 0.5 to 5 hour⁻¹ (n-butaneweight/catalyst weight).
 17. The process of claim 2, wherein thedehydroisomerization reaction is conducted at 450° to 600° C., at apressure of 200 mm Hg to 5 kg/cm² and with an hourly spatial speed of0.5 to 5 hour⁻¹ (n-butane weight/catalyst weight).
 18. The process ofclaim 3, wherein the dehydroisomerization reaction is conducted at 450°to 600° C., at a pressure of 200 mm. Hg to 5 kg/cm² and with an hourlyspatial speed of 0.5 to 5 hour⁻¹ (n-butane weight/catalyst weight). 19.The process of claim 16, wherein the temperature is 500° to 580° C., thepressure is 400 mm. Hg to 2 kg/cm², and the spatial speed is 2 to 4hour⁻¹.
 20. The process of claim 16, wherein the temperature is 500° to580° C., the pressure is 400 mm. Hg to 2 kg/cm², and the spatial speedis 2 to 4 hour⁻¹.
 21. The process of claim 18, wherein the temperatureis 500° to 580° C., the pressure is 400 mm. Hg to 2 kg/cm², and thespatial speed is 2 to 4 hour⁻¹.
 22. A process for the preparation ofisobutane by dehydroisomerisation of n-butane, comprising contacting,under dehydroisomerization conditions, gaseous flow of n-butane andhydrogen, at a temperature in the range of 450° C.-600° C., with a fixedcatalytic bed formed of a catalyst having a solid granular support ofporous gamma-alumina on the surface of which are deposited 0.1 to 1% byweight of platinum and 0.5 to 5% by weight of silica, and wherein saidcatalyst Is subjected to a reduction treatment before use indehydroisomerisation.
 23. A process for the preparation of isobutene bydehydroisomerisation of n-butane, comprising contacting, underdehydroisomerization conditions, gaseous flow of n-butane and hydrogen,at a temperature in the range of 450° C.-600° C., with a fixed catalyticbed formed of a catalytic system having a first catalyst formed of asolid granular support of porous gamma-alumina on the surface of whichare deposited 0.1 to 1% by weight of platinum and 0.5 to 5% by weight ofsilica and a second catalyst formed of porous gamma-alumina on thesurface of which is deposited a catalytic quantity of silica, andwherein said catalytic system is subject to a reduction treatment beforeuse in dehydroisomerisation.